Optical: systems and elements – Optical frequency converter – Dielectric optical waveguide type
Reexamination Certificate
1998-05-21
2001-03-27
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Dielectric optical waveguide type
C385S027000, C385S122000
Reexamination Certificate
active
06208455
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength converter for use in wavelength division multiplex (WDM) optical communication, and more particularly for use in high-speed WDM optical communication, and to a method for removing wavelength chirping from such a wavelength converter.
2. Related Arts
In recent years, WDM optical communication has been developed into a mass optical communication system. With this WDM optical communication system, each wavelength in a light signal is allocated to an individual communication channel, and therefore, in order for a signal to be transmitted or to be received from another channel, it is necessary for the wavelength of the original signal string to be converted into a wavelength that is suitable for a second channel. For conversion of a signal wavelength using a conventional optical device, a light signal must first be converted into an electrical signal, which then must be converted into a light signal having a different wavelength.
On the other hand, research has recently been performed employing a device that is so designed that it converts optical signal wavelengths directly, without converting them to electrical signals first. And there has been some discussion of proposed interactive devices, such as a symmetrical Mach Zender device and a deflected light isolation device. There has also been a report of the use as an optical switch of the symmetrical Mach Zender type wavelength converter (“Applied Physics Letters,” by S. Nakamura, et. al., vol. 67, pp. 2445) that has one of the same features as has a wavelength converter: it produces output signal pulses having wavelengths that differ from those of input signal pulses. This symmetrical Mach Zender type wavelength converter has the configuration shown in
FIG. 1
, in which two waveguides that can produce changes in nonlinear refractive indices are arranged along the respective arms of a Mach Zender interferometer. If a signal having a pulse width that is sufficiently shorter than &Dgr;t is input at time t
1
and time t
1
+&Dgr;t, a light pulse is output that has a first transition time t
1
and a last transition time t
1
+&Dgr;t. Further, a wavelength converter having the above described structure, with the exception that an optical delay circuit is arranged differently, is disclosed by S. Nakamura, the present inventor, in Japanese Unexamined Patent Publication No. Hei 7-199240.
The operating principle of an XPM type wavelength converter differs from that of a symmetrical Mach Zender wavelength converter in that it produces an output signal having substantially the same pulse as that of an input signal (“Journal of Lightwave Technology,” by T. Durhuus, et. al., vol. 14, pp. 942). However, the XPM wavelength converter, as well as the symmetrical Mach Zender wavelength converter, has a configuration in which two waveguides that produce changes in nonlinear refractive indices are arranged along the respective arms of a Mach Zender interferometer.
One problem arising from the use of these wavelength converters is that an output signal pulse produces a great deal of wavelength chirping. The wavelength chirping occurs because the changes in the nonlinear refractive indices at the waveguides alter the phase of a signal pulse light. A specific example of the wavelength chirping produced by the XPM wavelength converter is reported by Durhuus et al. in the above reference.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a wavelength converter that prevents the occurrence of wavelength chirping.
It is another object of the present invention to provide a wavelength converter that outputs a signal pulse that has no wavelength chirping and that has a frequency that is an integral multiple of that of an input signal pulse.
It is a further object of the present invention to provide a method for reducing or removing the occurrence of wavelength chirping in a wavelength converter.
To achieve the above objects, in one aspect, a wavelength converter according to the present invention comprises:
a waveguide for producing a change in a nonlinear refractive index;
a light interaction circuit that includes a light branching section and a light combining section;
a light delay circuit; and
a continuous-wave light(hereinafter referred to as a CW light) source,
wherein a difference (&Dgr;L
d
) in optical paths provided in the light delay circuit satisfies &Dgr;L
d
=v
g
T
o
/2, where T
o
indicates an input signal pulse interval and v
g
indicates a speed at which a signal pulse light proceeds through the light delay circuit.
A wavelength converter according to the present invention, in another aspect, produces a &pgr;/2 or greater change in a refractive index at the nonlinear waveguide.
A wavelength converter for reducing or removing the occurrence of wavelength chirping according to the present invention comprises a waveguide for producing a change in a nonlinear refractive index, a light interaction circuit including a light branching section and a light combining section, a light delay circuit and a sequential light source, whereby an interval T
o
between input signal pulses is so set that a condition &Dgr;L
d
=v
g
T
o
/2 is satisfied, with &Dgr;L
d
denoting a difference in optical paths provided in said light delay circuit and v
g
denoting a speed at which a signal pulse light proceeds through said light delay circuit.
The basic operation of the wavelength converter of the present invention controls transient changes in the phases of two different CW lights that proceed separately through the wavelength converter, and cancels out wavelength chirping before the two lights interact to produce an output pulse signal.
REFERENCES:
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patent: 6115520 (2000-09-01), Laskowski et al.
patent: 0875782 (1998-11-01), None
patent: 7-199250 (1995-08-01), None
patent: 8-334796 (1996-12-01), None
Zucchelli et al., “New Solutions For optical packet delineation and in synchronization optical packet switched networks,” ECOC '96. 22ndEuropean Conference on Optical Communication, (IEEE Cat. No. 96TH8217), Proceedings of European Conference on Optical Communication, Oslo, Norway, Sep. 15-19, 1996, pp. 301-304, vol. 3, XP002131871, 1996, Kjeller, Norway, Telenor, Norway, ISBN: 82-423-0418-1, * figure 5 *.
Stubkjaer et al., “Wavelength conversion devices and techniques,” Proceedings of the 1996 22ndEuropean Conference on Optical Communication, ECOC. Part 4 (of 6); Oslo, Norway, Sep. 15-19, 1996, vol. 4, 1996, pp. 33-40, XP002131872 Eur. Conf. Optc. Commun ECOC; European Conference on Optical Communicati0on, ECOC. 1996 IEEE, Piscataway, NJ, USA, Chapter: Converters based on optically controlled gates.
IDLER et al., “Chirp-free 2.5-GBIT/S Signal Generation By a Monolithic Mach-Zehnder Interferometer Wavelength Converter and Transmission Over 188 KM of Standard Fiber,” Optical Fiber Communication, New York, US, IEEE, 1995, pp. 73-74, XP000517653 ISBN: 0-7803-2654-7, * the whole document *.
Kazutaka et al.,Appl. Phys. Lett., 65 (19): pp. 2445-2447 (1994)(Nov.).
Terji Durhuus et al.,Journal of Lightwave Technology, vol. 14, No. 6, pp. 942-954 (1996)(Jun.).
S. Nakamura et al.,Appl. Phys. Lett., 62 (9): pp. 925-927 (1993)(Mar.).
K. Tajima et al.,Appl. Phys. Lett., 67 (25): pp. 3709-3711 (1995)(Dec.).
N.S. Patel et al., IEEE Photonics Technology Letters, vol. 8, No. 12, pp. 1695-1697 (1996)(Dec.).
Nakamura Shigeru
Tajima Kazuhito
Ueno Yoshiyasu
Lee John D.
NEC Corporation
Ostrolenk Faber Gerb & Soffen, LLP
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